Learning About Eclipses By The Light Of The Silvery Moon

Family life - family learning

December 12, 1991|By William F. Russell, Special To The Sentinel

There are two important celestial events that are coming up on successive days, a lunar eclipse and a solstice, and each can provide a great opportunity for us and our children to learn more about the wonders of our solar system.

In the early morning hours of Saturday, Dec. 21, a small part of our moon will pass through the shadow of the Earth. This ''partial eclipse of the moon'' is not as rare, nor will it be as impressive, as the solar eclipse we witnessed last July, but it's a wondrous event just the same.It also can serve as a teaching and learning vehicle to help prepare us for some more spectacular eclipses that will occur during the new year.

In order to acquaint your children with the mechanics of a lunar eclipse, it may be helpful to have a flashlight on hand, along with several common spheres, such as a marble, a Ping-Pong ball, a tennis ball or a grapefruit, that you can use to represent the Earth and moon.

By placing a marble or Ping-Pong ball on a table, and holding a lighted flashlight near the table's surface, you can create a long shadow that extends from the object and narrows like a cone down to a point. (This shadow is called the umbra, from the Latin word for shadow, which is the root for our word umbrella.)

The Earth casts this same type of shadow in space - 24 hours a day - because the Earth always is standing in the path of some of the light from the sun. This circular shadow is widest at the Earth's surface, and when people on the Earth rotate into the shadow, they refer to it as ''night.''

From the Earth this same shadow stretches 850,000 miles out into space, but the circle gets smaller and smaller before it narrows down to nothing. In all that immense distance, however, there is only one object that the shadow can fall upon. Every once in a while, our moon, which is whizzing around the Earth at about 2,280 mph, will cross the path of this shadow, and some or all of the sunlight falling on the moon's surface will be blocked out until the moon emerges from the shadow once again.

At 5 a.m. EST on Dec. 21, the very bottom part of the moon will touch the very top of this shadow, and by 5:33 a.m. almost 10 percent of the moon will be hidden, while the remainder will be bathed in bright sunlight.

By using the flashlight and two of the spherical objects, you can demonstrate why an eclipse can occur only during the time of a full moon. The moon has to be in line with the Earth and the sun in order to enter the Earth's shadow, and so anyone on the ''night'' side of the Earth would see the moon fully lighted until it touches that shadow.

When some ancient Greek thinkers, such as Pythagoras and Aristotle many centuries ago, saw this phenomenon we call a ''partial eclipse,'' they noticed that no matter where in the sky the eclipse occurred, the shape of the shadow on the moon was always curved in the arc of a circle. Through their reasoning and logic alone, therefore, they concluded that the Earth could not be flat, and must be a giant sphere.

Next week we'll discuss the other upcoming celestial event, the solstice.